The present invention relates in general to integrated circuits and components, such as may be employed in telecommunication circuits and the like, and is particularly directed to a new and improved transistor circuit for generating a programmable output parameter, such as a reference current that is controllably and precisely established by a user-programmable component (resistor) coupled therewith, without internal parameters of the circuit being subject to being modified (distorted) by the programming element.
FIG. 1 diagrammatically a conventional bipolar transistor circuit that may be incorporated into a variety of integrated circuits for generating a programmable current reference. For this purpose, a precision voltage element 10, such as a bandgap voltage reference device, is coupled between a (VCC) voltage supply rail 12 and the collector 21 of a bipolar (NPN) reference transistor 20. Transistor 20 has its emitter 22 coupled in circuit with a resistor 25, which is internal to the integrated circuit and is terminated at a reference voltage terminal (ground (GND)). An additional base current offset (NPN) transistor 30, whose collector 31 is tied to the VCC supply rail 12, has its emitter 32 coupled to the base 23 of transistor 20 and its base 33 coupled to the collector 21 of transistor 20. The base 23 of the reference transistor 20 is further coupled in common with the base 43 of an output transistor 40, the emitter 42 of which is coupled to a programming terminal 45 and the collector 41 of which is coupled to an output terminal 50. The programming terminal 45 is adapted to be coupled through a programming circuit element, such as a resistor 47, referenced to ground, while the output terminal 50 is used to supply an output current having a magnitude defined by the value of the programming resistor 47.
Since the intended functionality of the circuit of FIG. 1 is to generate a reference current at output terminal 50 that is precisely established by the value of the programming resistor 47 in proportion to the bandgap voltage of the precision bandgap voltage reference device 10. The insertion of the base current offset transistor 30 serves to reduce the effect of base current errors. In operation, with a programming resistor 47 of some value Rprog coupled between the programming terminal 45 and ground, the following voltage loop equation (1) may be defined:
(I10/xcex1n*R25+Vbe20(@I10/xcex1n)=(I50/xcex1n)*R47+Vbe40(@I10/xcex1n)xe2x80x83xe2x80x83(1)
Since, however, I10=Vbandgap/Rintxe2x80x83xe2x80x83(2)
where Vbandgap is the bandgap voltage and Rint is a resistor internal to the integrated circuit which matches the resistor 25 (also internal to the integrated circuit, as noted above), then equation (1) may be rewritten as:
Iout=I50=(1/R47)*(Vbandgap*R25/Rint+xcex94Vbe30,40)xe2x80x83xe2x80x83(3)
where xcex94Vbe30,40 is the difference between the base-emitter voltage drops of transistors 30 and 40. This base-emitter voltage difference may be significant, even where transistors 30 and 40 are designed to have identical geometries, since there is no way to predict their relative current densities, which are a function of the programming resistor 47. As a result, the internal parameters of the circuit of FIG. 1 are subject to being influenced by the programming element, so that the output current generated at terminal 50 cannot be accurately programmed as desired.
In accordance with the present invention, this problem is effectively obviated by a relatively simple circuit architecture whose internal parameters are effectively independent of the programming element. Like the architecture of FIG. 1, the present invention makes use of a bandgap voltage reference device incorporated in the integrated circuit. The bandgap voltage reference device generates a reference current IK*temp that is proportional to temperature (in degrees Kelvin (xc2x0K)). This bandgap based reference current IK*temp is supplied through the collector-emitter path of a reference transistor coupled in circuit with a reference resistor terminated at a reference voltage terminal. The reference resistor has a geometry that effectively matches that of the internal bandgap device""s resistance and has a value such that the sum of the base-emitter voltage drop across the reference transistor and the voltage drop across the reference resistor resulting from the reference current IK*temp is equal to the bandgap voltage.
The base of the reference transistor is further coupled in common with the emitter of an output transistor and to a programming terminal, that is adapted to be coupled to a programming resistor. The base of the output transistor is coupled to the collector of the reference transistor, while the collector of the output transistor is coupled to an output terminal, from which a programmed current is to be supplied in accordance with the value of the programming resistor.
The loop equations for are such that the output current is effectively definable as the ratio of the bandgap voltage Vbandgap to the programming resistor, and is not affected by the base-emitter voltage drops of the reference and output transistors, as in the conventional bandgap-based circuit (shown in FIG. 1). This means that the output current supplied by the invention may be programmed in accordance with the precision of the integrated circuit""s internal bandgap device and the tolerance of the programming resistor without significant first order errors.